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This Stackoverflow posting has a rundown on workstation hardware. Media production types are a major user of such hardware. You may find something of interest there.

If you're doing anything with video, make sure your disk subsystem is fast enough. Software RAID on Windows in general and XP in particular is pretty rubbish. You might want to benchmark a H/W SATA RAID controller. Some will do stripe sizes up to 1024K, so you could get pretty good streaming performance from a RAID-0 of (say) 4 inexpensive SATA drives. A basic 4 or 8 drive controller from Areca, 3Ware, LSI or Adaptec is just a few hundred dollars. The whole disk subsystem would be just a fraction of the cost of Adobe Creative Suite.

The more memory the better. Single-socket PC's tend to max out at 4GB or sometimes 8GB
of RAM. Two-socket machines tend to be more expensive. Look into something with intel 'Nelahem' processors like a current-gen Mac Pro or HP (Z400, Z600 or Z800). These processors are (a) very fast and (b) can take quite a lot of memory, even on single socket models. Check out pricing on third-party memory as the manufacturers of this kit will charge a fairly steep price for bits. 16GB or more is quite feasible on these machines without breaking the bank.

Note that you will need a 64-bit O/S and 64-bit apps to use this RAM. Hope you don't mind Vista.

If you have GPU support on your software, consider getting something like a NVidia Tesla C1060 to accelerate this. You will need a machine with a suitable motherboard and power supply. Off-the shelf, a HP Z800 or a similar machine would do this if you have the money but they are quite expensive.

If you want to do it on the cheap, there are a range of single-socket workstation and gaming boards available from most motherboard manufacturers that a custom white-box outfit could put into a machine. Also, older workstation models like a XW8600 or XW9400 can be bought off ebay fairly cheaply. They are not quite as fast as the newer Nelahem boxes but the big ones have two pcie-x16 slots and beefy power supplies that can support a high-spec video card and a second Tesla card. They also have the advantage over generic white-box kit that somebody has put some thought into the airflow in the case and the machine is rated by the manufacturer to support this type of configuration.

Finally, consider getting a calibrated monitor like these ones from Barco, HP or Eizo. If you're doing work for production video or print you can tune these monitors to replicate the characteristics of your target medium. This type of monitor can pay for itesl

I alas do not get the luxury of a desktop and have to live with what I can cram into a laptop I can carry.

Video, with the highest resolution possible is critical. My Dell D830 is running at 1920X1200 and I could not live with any less, and would love more. (15" too! Not the 17" monstrosity! I love it!)

Next most important is hard drive speed. (I take 4GB RAM at a minimum as a given, RAM is cheap, as much as the system will take!).

I foolishly traded in my 100GB 7200 rpm SATA drive, for a 300GB 5400 rpm SATA drive and I regret it daily. I needed the extra space, but the performance hit surprised me!

Spend what it takes for the fastest drive you can afford! Consider an SSD for a boot partition. Disk performance is crippling, when it is the bottleneck. And it is, a surprising amount of the time!

Run Process Explorer, add on I/O Read counters, and look at how many read bytes, your AV software had done! I was running for about 2 weeks between reboots, and the counter for Symantec Endpoint was on the order of 19 TB!!! Now I do a LOT of 'stuff' but that seems extreme even for me!

RAM, RAM and more RAM! Those are all RAM-hungry monster apps (I include WinXP in that), so before spending thousands on a new machine or hundreds on a new graphics card, I recommend trying a few tens on some more good quality main memory (which you can always re-use in a new machine later).

Number one on the list is RAM. It is by far your cheapest upgrade option so put the maximum amount in that the motherboard can handle. XP / Vista x32 will truncate this down to somewhere between 3 and 3.5GB; however, every little bit helps.

Second, put two of the fastest harddrives you can find in it. One for a scratch disk, the other as the primary. Assuming you don't want to use SCSI or SAS, some of the 1TB and 1.5TB drives actually out perform Western Digital's Raptors in terms of transfer rates. You might even consider using RAID 0 to eak a bit more out of them. If you do this, make sure you have a good quality RAID controller (not the one on the motherboard). As a side note, do NOT use SSD disks. They start off fast but slow down over time when attempting to overwrite old files. Stick with the regular ones.

Third, that 8400GS is not a good video card. It was sold as a low level card when it first came out, so get a better one. The latest versions of Adobe, and just about every other piece of graphic software, can actually put the graphic processor to good use. Consider spending between $300 and $1200 here depending on what you can afford.

Finally, that processor is a bit long in the tooth. Get a new motherboard / processor.